Why not use standard panel unit root test for testing PPP

In this paper we show the consequences of applying a panel unit root test that assumes independence between the cross-sections when testing for a purchasing power parity relationship. The distribution of the tests investigated, including the IPS test of Im et al (2003), are influenced by a common stochastic trend which is usually not accounted for. The result is that the empirical size tends to one with the number of cross-sections. Hence, it is of crucial importance to account for this cross-sectional dependency.